create any hazards. A potential hazard could be the airplane sliding toward other airplanes due to the higher power settings required. If run-ups are performed during compliance demonstration, these procedures should be incorporated as limitations in the flight manual.
2. Higher power settings
Higher power settings, which could result in increased snow ingestion, associated with taxi/hold ground operations.
3. Takeoff run to takeoff speed
Airplanes with identified sites of possible hazardous snow accumulation and all inlets with bypass ducts (for example, typical turbopropeller inlets), should consider a takeoff run to takeoff speed. This condition is considered critical since—
a. Accumulated snow may come loose and the engine can ingest it in this condition;
and
b. The static, idle point will not provide the ram effects that create bypass flow for bypass ducts.
4. Takeoff climb
Airplanes with identified sites of possible hazardous snow accumulation should consider a takeoff climb. This condition is considered critical because accumulated snow may come loose and the engine can ingest it in this condition.
5. Extended in-flight operations
Conditions such as holding patterns should be considered.
6. Operation when engine rotor speeds are low
Operations with low engine rotor speeds, such as during descent from high altitudes, should be considered. An engine is highly susceptible to snow/ice accretion during this condition.
Is a flight test required for all these operational conditions?
The preceding conditions are operational considerations and not meant to require flight test at all the conditions. Each installation may have different critical operational considerations and only the critical conditions may need flight testing.
When appropriately substantiated by the applicant, some of the conditions can be, and have been, simulated and accepted by the FAA. For example, for a turbopropeller engine 129
that incorporates an inlet screen that precludes the ingestion of hazardous quantities of materials, the critical concern may only be the effect of snow accumulation and release from the inlet and screen. In this case, the inlet, bypass duct, inlet screen, and so forth, could be blocked to simulate snow accumulation on an identified area of concern. Since accumulation during dynamic operation would be simulated, the effects of snow ingestion could be determined through ground tests (for example, effects of operability on items such as reverse flow). Such methodologies need to be substantiated by means such as design analysis, operational review, tunnel tests, icing tests, and so forth, and coordinated early with the FAA.
How is the actual snow amount quantified for testing?
When testing in ”falling and blowing snow” the actual snow amount is often difficult to quantify. The FAA Aeronautical Information Manual (AIM), an official FAA guide to basic flight information and air traffic control procedures, may be used as guidance for what constitutes falling and blowing snow. According to the AIM, paragraph 7-1-18, heavy snow, which is representative of what may be expected in operation, is defined as visibility of 1/4 mile or less as limited by snow (not snow and fog). These conditions are usually indicative of the wet snow environment necessary for test. When using the 1/4 mile or less visibility for test, including flight tests, this value can be determined using ground conditions. Useful information on snow characterization can also be found in FAA Report DOT/FAA/AR-97/66, Snow and Ice Particle Sizes and Mass Concentrations at Altitudes Up to 9 km (30,000 ft.). Advisory Circular 29-2C (Certification of Transport Category Rotorcraft), section AC 29.1093, paragraph c(4)(iv), also provides information on snow quantification, including desired snow concentration, which is acceptable for use on part 23 airplanes. Whatever method is used to characterize the snow, as mentioned earlier, the design consideration that has been found to be critical is snow that accumulates on surfaces subject to impingement. Therefore, the applicant should address this consideration when choosing the appropriate snow environment.
What temperatures are considered appropriate for testing?
The applicant is responsible for defining the critical ambient temperatures for snow tests.
The primary consideration is to demonstrate operability in a snow environment that is critical as far as snow accumulation on exterior and interior areas of impingement (for example, wet, sticky snow). However, for a snow environment indicative of a representative concentration expected for the airplane, temperature is also an important consideration.
Typically, in natural conditions, a temperature range between 25 and 34 degrees Fahrenheit has been found conducive to the heavy snow environment. However, colder temperatures may be critical to some configurations. For example, in some installations, colder exterior surfaces may be by-passed, with snow crystals sticking to partially heated interior inlet surfaces, leading to melting and refreeze. Company developmental tests or experience with similar induction systems may be used to determine critical conditions.
Is it difficult to locate and maintain the desired conditions for the test?
Snow concentration corresponding to the visibility prescribed is often extremely difficult to locate naturally and it is often difficult to maintain the desired concentrations for the duration of testing. Because of this, it is likely that exact target test conditions will not be achieved for all possible test conditions. Those involved in certification must exercise reasonable engineering judgment in accepting critical test conditions and alternate approaches, with early coordination between the applicant and the FAA addressing these realities.
Is it possible to use artificially produced snow for the test?
Artificially produced snow is an excellent developmental tool and has been used successfully to show potential problem areas and critical test points. When the desired natural snow concentration is not found, artificial means may be used to supplement the snow amount. However, when snow testing is required, the use of simulated snow is normally not used as the sole means of compliance. The desired heavy snow environment produces ”wet, sticky snow,” which accumulates on unheated exterior and interior surfaces subject to impingement. Most artificial means (for example snow blowers) produce snow pellets that are dissimilar to the snowflakes associated with ”wet, sticky snow.” Also, simulated snow produced indoors does not accumulate moisture from snowfall as seen in naturally created snow, with critical temperatures for simulated snow varying significantly from natural snow. Therefore, quantification of artificially produced snow for critical conditions can be difficult and subjective. If artificial means is proposed as a means of compliance, the applicant should provide data and substantiation on how the artificial means will effectively simulate the critical, desired operational consideration.
What is the difference between falling and blowing snow?
The concentration of snow entering the inlet in blowing snow will normally exceed the amount in falling snow; hence, the need to address ”blowing snow.” The location of the inlets should be considered to determine critical directions of blowing snow in relation to snow accumulation on impingement surfaces. Snow blowing in excess of 15 knots is the appropriate compliance condition. Means such as use of another airplane's propeller, taxiing the airplane in excess of 15 knots, and so forth, may be used to simulate blowing.
Is it possible to use flight manual limitations to comply with § 23.1093(b)(1)(ii)?
The only possible flight manual limitation that may be acceptable would be prohibitions for ground operations such as taxi, takeoff, engine runs, and so forth. As with all environmental considerations, such as rain, ice, hail, lightning, and so forth, operation in snow is considered an unavoidable, meteorological hazard that must be addressed including flying into snow after deployment.
What should be considered relative to complying with the § 23.1093(b)(2) requirements?
The requirement of § 23.1093(b)(2), incorporated by Amendment 23-15, addresses running the engine in an atmospheric condition commonly referred to as ”ice fog.” The engine is run at idle power in this condition to ensure no ice build-up that produces adverse engine operational characteristics (for example, no surges or adverse power losses). The § 23.1093(b)(2) requirement is as an extension of the 14 CFR part 25, Appendix C, icing envelope addressed in §§ 23.1093(b)(1)(i) and 23.1419. Therefore, the methodologies and analysis used for compliance with § 23.1093(b)(1)(i) can be extended for § 23.1093(b)(2) compliance.
It is often difficult to encounter all the ambient conditions required by § 23.1093(b)(2);
therefore, when testing, one or more of the conditions is typically simulated. For example, a common and acceptable method of compliance is using water spray devices to simulate the water conditions required while testing at the required ambient temperature conditions. Other manufacturers have used thermal analysis combined with dry air tests using ice shapes/simulated blockage to demonstrate compliance, which is also acceptable if properly substantiated.
The rule allows an engine run-up periodically to higher power settings to shed ice. As with snow testing, if run-ups are performed during compliance demonstration, then these procedures should be incorporated as limitations in the flight manual. Also, before runups are accepted, the practicality of the procedures should be evaluated.
Related Material
FAA Aeronautical Information Manual (AIM)
FAA Report DOT/FAA/AR-97/66, Snow and Ice Particle Sizes and Mass Concentrations at Altitudes Up to 9 km (30,000 ft.)
AC 23.1419-2B, Certification of Part 23 Airplanes for Flight in Icing Conditions
AC 20-73, Aircraft Ice Protection
AC 29-2C, Certification of Transport Category Rotorcraft
Paragraph 23.1093(c)
ADC Cross-reference: 5.45(c)
No specific policy is available as of December 31, 2002.
23.1095 Carburetor deicing fluid flow rate (Original)
CAR 3 Cross-reference: None
ADC Cross-reference: None
Paragraph 23.1095(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.1095(b)
No specific policy is available as of December 31, 2002.
23.1097 Carburetor deicing fluid system capacity (Original)
CAR 3 Cross-reference: None
ADC Cross-reference: None
Paragraph 23.1097(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.1097(b)
No specific policy is available as of December 31, 2002.
23.1099 Carburetor deicing fluid system detail design (Original)
CAR 3 Cross-reference: None
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
23.1101 Induction air preheater design (Amendment 23-43)
CAR 3 Cross-reference: 3.610
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
What is the purpose of the induction air preheater?
Some carburetor and fuel injection system designs do not experience icing. Designs that are susceptible to icing typically incorporate a means to preheat the air supply entering the system. The purpose of this system is to add sufficient heat to replace the heat lost due to vaporization of the fuel such that the mixing chamber temperature cannot drop to the freezing point of water. This is typically accomplished with a system consisting of an engine exhaust heated jacket surrounding the induction duct, which serves to preheat the inducted air. There is a control installed in the flight deck for adjusting the preheater valve such that the system may be operated only when required to prevent ice formation.
Paragraph 23.1101(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.1101(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.1101(c)
No specific policy is available as of December 31, 2002.
23.1103 Induction system ducts (Amendment 23-43)
CAR 3 Cross-reference: 3.611
ADC Cross-reference: 5.47
Paragraph 23.1103(a)
ADC Cross-reference: 5.47(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.1103(b)
ADC Cross-reference: 5.47(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.1103(c)
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
Paragraph 23.1103(d)
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
Paragraph 23.1103(e)
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
Paragraph 23.1103(f)
ADC Cross-reference: None
No specific policy is available as of December 31, 2002.
23.1105 Induction system screens (Amendment 23-51)
CAR 3 Cross-reference: 3.612
ADC Cross-reference: 5.48
Paragraph 23.1105(a)
ADC Cross-reference: 5.48(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.1105(b)
ADC Cross-reference: 5.48(b)
No specific policy is available as of December 31, 2002.
Paragraph 23.1105(c)
ADC Cross-reference: 5.48(c)
No specific policy is available as of December 31, 2002.
Paragraph 23.1105(d)
ADC Cross-reference: 5.48(d)
No specific policy is available as of December 31, 2002.
23.1107 Induction system filters (Amendment 23-51)
CAR 3 Cross-reference: None
ADC Cross-reference: None
This guidance may be used for Airship Certification as applicable.
Paragraph 23.1107(a)
What should be considered in the design of induction system filters to comply with § 23.1107?
The filter media, gasket materials, and adhesives used in the induction system should be resistant to fuel, oil, water, cleaning materials, and other engine/airplane fluids in the powerplant installation. They should be flame resistant and should not deteriorate from exposure to engine compartment temperatures.
Paragraph 23.1107(b)
What design features should be considered for filter installations?
Design the filter installation to preclude the release of pieces of the filter and/or gasket material into the intake airstream. These failures are critical as they can result in a power loss. Consider the performance of the filter under the operating conditions of the installation such as rain impingement at airplane speeds. One methodology is to provide screens or other restraints to prevent failed pieces from moving downstream and interfering with the fuel metering components.
23.1109 Turbocharger bleed air system (Amendment 23-42)
CAR 3 Cross-reference: None
ADC Cross-reference: None
Paragraph 23.1109(a)
No specific policy is available as of December 31, 2002.
Paragraph 23.1109(b)
No specific policy is available as of December 31, 2002.